Diversity in motor skills is achieved in large part by a process that selects for the activation, or recruitment, of motor units that are specialized with respect to their tension producing characteristics. A major aim of this proposal is to continue investigation of the patterns observed in this selection process, i.e. to study recruitment order. "Size" and type characteristics will be measured from motor units recruited in sequence in segmental reflexes. These characteristics are central to the two most prominant hypotheses proposed to explain recruitment order, and their simultaneous measurement will help to resolve the current controversy over which scheme actually produces orderly recruitment. Evidence to support the hypothesis favored by this study will be obtained in a parallel investigation using intracellular recording and stimulation techniques to measure biophysical properties of motoneurons, their monosynaptic connections and their adjunct muscle fibers. As a result of these studies, the process that accounts for the appropriate matching of motor capabilities with motor tasks will be much better understood. With the functional organization of normal motor units clearly described, the project will proceed to meet another major aim, namely to describe some chronic effects of motor nerve transection. Very little information is available concerning the recoverability of recruitment order. A single unconfirmed study demonstrates that volitional recruitment of motor units belonging to a reinnervated muscle in the human hand is disordered. Other studies indicate changes in motor-unit functional properties that may account for the abnormal recruitment order. Both methods used to study normal motor units as described above will also be applied in an investigation of the chronic changes in recruitment order and in biophysical properties of motor units in a muscle self-reinnervated after nerve section. These data will provide a more complete prognosis of the potential for recovery of normal motor function in victims of nerve injury. Furthermore, they will provide a much more complete picture of the extent to which motor unit properties are restored following nerve transection.